The present invention relates to titanium metallurgy. The invention relates more particularly to processes for treating titanium alloys to enhance physical and mechanical properties of the alloys, such as ultimate tensile strength, notched tensile strength, and fatigue resistance, particularly at cryogenic temperatures.
Titanium alloys are frequently used in aerospace and aeronautical applications because of the superior strength, low density, and corrosion resistance of titanium alloys. Titanium and its alloys exhibit a two-phase behavior. Pure titanium exists in an alpha phase having a hexagonal close-packed crystal structure up to its beta transus temperature (about 1625xc2x0 F.). Above the beta transus temperature, the structure changes to the beta phase having a body-centered-cubic crystal structure. Pure titanium is quite weak and highly ductile, but can achieve high strength and workable ductility when alloyed with other elements. Certain alloying elements also affect the behavior of the crystal structure, causing the alloy to behave either as an alpha or near-alpha alloy or as an alpha-beta alloy at room temperature. Alpha-beta alloys are made by adding one or more beta stabilizers, such as vanadium, which inhibit the transformation from beta to alpha and depress the beta transus temperature such that the alloy exists in a two-phase alpha-beta form at room temperature. Alpha alloys are made by adding one or more alpha stabilizers, such as aluminum, which raise the beta transus temperature and stabilize the alpha form such that the alloy is predominately in the alpha form at room temperature.
Two basic types of titanium alloys are currently in use in the rocket propulsion industry: Ti-6-4, an alpha-beta alloy consisting principally of about 6 percent aluminum, 4 percent vanadium, and the balance titanium and incidental impurities;
and Ti-5-2.5, a near-alpha alloy consisting principally of about 5 percent aluminum, 2.5 percent tin, and the balance titanium and incidental impurities. The Ti-6-4 alloy is more readily available and is more easily processed to final form than the Ti-5-2.5 alloy, making, Ti-6-4 much less costly than Ti-5-2.5.
Very low-temperature applications, such as for hydrogen fuel pumps or the like, impose severe restrictions on the types of alloys that can be used, primarily because the notch sensitivity of an alloy can be degraded to unacceptable levels at such temperatures. Of the currently available commercially produced alloys, Ti-5-2.5 ELI (Extra Low Interstitial grade processed to have reduced incidence of interstitial impurities) is currently the alloy of choice for cryogenic temperature applications because of its relatively high ultimate strength (on the order of 210 ksi) and its relatively high notch tensile ratio or NTR (on the order of 1.1) at liquid hydrogen temperatures of about 20K. The NTR is defined as the ultimate tensile strength of a notched test specimen divided by the ultimate tensile strength of a smooth test specimen, and is a standard measure of the notch sensitivity of a material. The more common, stronger, and less costly Ti-6-4 ELI alloy is known to have poor ductility and be notch sensitive (i.e., its NTR is less than 1.0) at cryogenic temperatures of 77K and below, and thus is a less favorable choice.
It would be desirable, however, to be able to use the stronger Ti-6-4 alloy in cryogenic and other applications, rather than the Ti-5-2.5 alloy, because Ti-6-4 is significantly less costly. Additionally, there is typically a very long, lead time for purchase of Ti-5-2.5 ELI because there currently are only two known significant domestic users of this alloy. Accordingly, use of Ti-6-4 would enable quicker turnaround times. Furthermore, it would be desirable to provide a titanium alloy having improved ultimate strength compared to both Ti-5-2.5 and standard Ti-6-4, and having an acceptable NTR, preferably at least 1.0, at cryogenic temperatures. To achieve these ends, however, a non-standard processing of the standard Ti-6-4 alloy would be required in order to improve the strength and NTR at cryogenic temperatures.
It is known from the Hall-Petch relationship in physical metallurgy that decreasing the grain size results in an increase in strength. There is no known generally applicable correlation between grain size and NTR in alpha-beta titanium alloys, and very little data are available on how the properties of alpha-beta alloys behave as a function of grain size at cryogenic temperatures. There are some data to suggest, however, that at least in steels, an equiaxed grain size reduction can lead to both an increase in strength and an increase in fracture toughness.
Standard mill practice for Ti-6-4 bar calls for forging to occur at a temperature where the alloy is in the 2-phase alpha-beta field. The primary alpha that exists at these temperatures, typically in the range of 1600 to 1750xc2x0 F., pins the beta grains during the deformation and leads to an initial grain size refinement. Following forging, the alloy is cooled to room temperature, which results in the decomposition of the high temperature beta grains to a lenticular mixture of alpha and beta through nucleation and growth processes. Thus, the final microstructure consists of relatively large xe2x80x9cprimaryxe2x80x9d alpha grains, on the order of 10 to 50 xcexcm, and a fine mixture of alpha and beta plates whose scale is dependent on cooling rate (i.e., finer as the cooling rate increases).
One method for attaining finer grain sizes would be to use dynamic recrystallization during hot working. This is the process that leads to the initial refinement of the beta grains during conventional forging of alpha-beta alloys described above. However, the alpha grains do not change size during conventional forging and they do not undergo recrystallization with increased strain. Accordingly, it is impossible to attain a uniform fine grain size with the conventional forging process for Ti-6-4 because of the presence of the primary alpha grains.
The present invention provides a unique titanium alpha-beta alloy and a process for treating an alpha-beta titanium alloy, such as Ti-6-4, which leads to a high ultimate strength and notch tensile ratio of 1.0 or greater at cryogenic temperatures. The process is based on the unexpected discovery that a high strength and an optimum notch tensile ratio at cryogenic temperatures are attained by a microstructural arrangement of equiaxed alpha grains and a beta phase predominately in the form of a non-equiaxed distribution surrounding the alpha grains, the microstructure having a maximum grain size of about 5 to 10 xcexcm, and the volume fraction of alpha being about 75 to 85 percent. Grain sizes below and above the 5 to 10 xcexcm scale lead to less than optimum notch tensile ratios. The required microstructure cannot be achieved using conventional titanium processing techniques.
In accordance with the present invention, a billet of alpha-beta titanium alloy is processed by first causing a transformation of the alloy to a substantially single-phase beta microstructure, then causing a martensitic transformation of the single-phase beta microstructure to produce a fine platelet alpha-beta microstructure. Thereafter, the billet is isothermally forged at a temperature about 300xc2x0 C. below the beta transus temperature of the alloy so as to attain a fine equiaxed microstructure such that a maximum grain size is on the order of about 2-5 xcexcm. After forging, the billet is aged at a temperature slightly below the beta transus temperature, preferably about 25xc2x0 C. to 75xc2x0 C. below the beta transus temperature, for a period of time sufficient to grow the refined microstructure such that a maximum grain size is on the order of about 5-10 xcexcm. Preferably, for Ti-6-4 ELI alloy having a beta transus of about 1000xc2x0 C., the billet is aged at about 925xc2x0 C. to about 975xc2x0 C. for about 30-60 minutes so as to grow the scale of the refined equiaxed microstructure by a factor of about 2.
When applied to a conventional Ti-6-4 ELI alloy, the process in preferred embodiments leads to notch tensile ratios of greater than 1.0 and ultimate tensile strengths of 240-250 ksi at temperatures of 4K and 20K. Furthermore, the resulting alloy has been found to have an improved high-cycle fatigue resistance at 4K relative to conventionally processed Ti-5-2.5 alloy.
In accordance with a preferred embodiment of the invention, the transformation to the substantially single-phase beta microstructure is accomplished by solution treating the billet at a temperature near or above the beta transus temperature of the alloy. For example, for Ti-6-4, which has a beta transus temperature of about 1000xc2x0 C., the billet is solution treated at a temperature in a range from about 990xc2x0 C. to about 1020xc2x0 C. for about 30 minutes.
The martensitic transformation of the beta alloy is accomplished preferably by cooling the billet at a rate in excess of air cooling to a temperature substantially below the beta transus temperature. For example, the billet can be quenched to about room temperature, such as by quenching in a liquid coolant, to induce a transformation of the single-phase beta microstructure to a predominately martensitic microstructure.
The isothermal forging operation is an important aspect of the process, enabling refinement of the fine platelet structure that results from the martensitic transformation. As noted above, the isothermal forging is conducted at a temperature substantially lower than the beta transus temperature, preferably about 300xc2x0 C. lower than the beta transus. For Ti-6-4 alloy, the forging is carried out preferably at about 700xc2x0 C. Advantageously, the billet is isothermally forged at a strain rate not greater than about 0.10 in/in/second. The total strain produced preferably should be in a range from about 0.5 to 0.8. For Ti-6-4, the total strain more preferably should be in a range from about 0.6 to 0.7.
A preferred process in accordance with the present invention has been used to treat conventional Ti-6-4 ELI alloy, leading to notch tensile ratios in excess of 1.0 and significant improvements in strength over both conventional T-6-4 ELI and Ti-5-2.5 ELI at cryogenic temperatures. It is anticipated, however, that the process should be advantageous for any alpha-beta titanium alloy.